Xenobiotics such as drugs and carcinogens as well as endobiotics such as steroids and fatty acids are metabolized by the mixed function oxidase systems. Cytochrome P-450 is the key component of mixed function oxidases and the type and quantity of specific forms of cytochrome P-450 is the key components of mixed function oxidases and the type and quantity of specific forms of cytochrome P-450 determine the disposition of a particular substrate. MAbs are specific probes for particular antigenic determinants and are useful tools for identification of particular isoenzymes. Myeloma cells were hybridized with spleen cells of mice immunized with purified human placenta mitochondorial cytochrome P-450. We obtained 25 independent hybridomas producing MAbs to the human placenta mitochondrial cytochrome P-450. Placenta microsomal and mitochondrial cytochromes P-4500 play important roles in steroid metabolism. In addition 3-methylcholanthrene inducible cytochromes P-450 of rats are also inducible in human placenta by smoking. Since the patterns of cytochromes P-450 present in human liver, placenta and other organs can be phenotyped with specific MAbs, the roles and interaction of different forms of cytochromes P-450 will be examined as possible markers of individual differences in drug metabolism and sensitivity to environmental carcinogens.